Diesel fuel ranks second only to gasoline as a fuel for internal combustion engines. Trucks, buses, tractors, locomotives, ships, power generators, etc. are examples of devices that use diesel fuel. Passenger cars and sport utility vehicles are another area of potential growth for the use of diesel engines that can provide improved fuel efficiency, especially where high torque at relatively low rpm is desired.
Diesel fuel is principally a blend of petroleum-derived compounds called middle distillates (heavier than gasoline but lighter than lube oil). Diesel fuel is designed to operate in a diesel engine, where it is injected into the compressed, high-temperature air in the combustion chamber and ignites spontaneously. This differs from gasoline, which is pre-mixed with air and ignited in a gasoline engine by the spark plugs. D2 diesel fuel conforms to specification D 975 set by the American Society for Testing and Materials (ASTM).
Unlike gasoline engines that operate by spark ignition, diesel engines employ compression ignition. In order to avoid long ignition delays resulting in rough engine operation, as well as to minimize misfiring and uneven or incomplete combustion which results in smoke in the exhaust gases that causes a major environmental problem, it is highly desirable to improve the burning quality of diesel fuels to minimize environmental pollutants such as hydrocarbons, carbon monoxide, particulate matter (commonly called soot), etc.
Cetane is an alkane molecule that ignites very easily under compression, so it is assigned a cetane number (CN) of 100. In general, the cetane number (CN) depends primarily on its hydrocarbon composition. Saturated hydrocarbons, particularly those with straight, open chains, have relatively high cetane numbers, whereas unsaturated hydrocarbons have relatively low cetane numbers. All other hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. The cetane number therefore measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions. Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components. Cetane improvers act to increase the effective cetane number of the fuel.
It is necessary to recognize that the relationship between the CN of diesel fuel and its performance cannot be equated in any way to the octane number of a gasoline and its performance in a spark-ignition engine. Raising the octane number allows an increase in the compression ratio and thus provides increased power and fuel economy at a particular fuel load. In contrast, in diesel engines, the desired CN provides good ignition at high loads and low atmospheric temperature. High cetane fuels eliminate engine roughness and diesel knock, allow engines to be started at lower temperatures, provide faster engine warm-up without misfiring or producing smoke and reduce formation of harmful deposits. On the other hand, too high cetane fuels can result in incomplete combustion and exhaust smoke due to too brief of an ignition delay which does not allow proper mixing of the fuel and air.
Commercial diesel fuels have CN numbers of at least 40. The suitable diesel fuel has appropriate volatility, pour and cloud point, viscosity, gravity, flash point and contain only small but tolerable levels of sulfur. It is also important that carbon, residue formation and ash content should be kept low.
During the normal course of operation, diesel engines often develop carbon deposits on the walls of their cylinders due to incomplete combustion of fuel. These deposits can increase engine wear and, because of friction induced by the deposits, decrease engine efficiency. Incomplete fuel combustion can also lead to the environmentally harmful emission of particulate materials, also referred to as soot. Thus, fuel additives that increase fuel combustion, protect the cylinder walls of diesel engines, and decrease engine friction, resulting in greater fuel efficiency, are highly desirable.
Sanduj a et al., U.S. Pat. No. 6,645,262, the disclosure of which is incorporated herein by reference, describes liquid hydrocarbon fuel concentrates, including low-sulfur diesel fuel concentrates, that include a suspension of particulate boric acid for the purpose of increasing lubricity and reducing engine wear.
Olah, U.S. Pat. No. 5,520,710, the disclosure of which is incorporated herein by reference, describes diesel fuel additives that are dissolved in the fuel and homogeneously distributed and include a dialkyl, alkyl-cycloalkyl, or dicycloalkyl ether compound together an alkyl or dialkyl peroxide compound for the purposes of enhancing cetane numbers and improving fuel combustion.
Peters et al., U.S. Pat. No. 6,158,397, the disclosure of which is incorporated herein by reference, describes a process for reducing soot in diesel engine exhaust gases wherein a fluid containing a peroxide compound, preferably aqueous hydrogen peroxide, is separately fed into the combustion chamber after the start of the injection and combustion of the fuel, preferably following the combustion phase.
Cunningham, U.S. Pat. No. 5,405,417, the disclosure of which is incorporated herein by reference, describes a fuel composition comprising a middle distillate base fuel having a sulfur content of less than 500 ppm and a clear cetane number in the range of 30 to 60, and a minor amount of at least one peroxy ester combustion improver such as t-butyl peroxyacetate dissolved therein.
Olsson et al., U.S. Pat. No. 5,105,772, the disclosure of which is incorporated herein by reference, describes a process for improving combustion in an engine that comprises: injecting a liquid composition that includes a peroxide or a peroxo compound into an engine combustion chamber, and passing a portion of the composition through the exhaust outlet valve as the engine goes from the exhaust phase to the intake phase, the passing occurring during the step of injecting.
Mellovist et al., U.S. Pat. No. 4,359,969, the disclosure of which is incorporated herein by reference, describes a method of improving fuel combustion that comprises: introducing a liquid composition consisting essentially of 1-10% hydrogen peroxide, 50-80% water, and 15-45% of a C1-C4 aliphatic alcohol, all by volume, in the form of fine droplets into the air intake manifold of an engine, where the droplets mix with air or fuel-air mixture prior to entering the combustion chamber. Preferably, the liquid composition also contains up to 5% of a thin lubricating oil and up to 1% of an anticorrosive.
Kracklaurer, U.S. Pat. No. 4,389,220, the disclosure of which is incorporated herein by reference, describes a method of conditioning diesel engines in which a diesel engine is operated on a diesel fuel containing from about 20-30 ppm of dicyclopentadienyl iron for a period of time sufficient to eliminate carbon deposits from the combustion surfaces of the engine and to deposit a layer of iron oxide on the combustion surfaces, which layer is effective to prevent further buildup of carbon deposits. Subsequently, the diesel engine is operated on a maintenance concentration of from about 10-15 ppm of dicyclopentadienyl iron or an equivalent amount of a derivative thereof on a continuous basis. The maintenance concentration is effective to maintain the catalytic iron oxide layer on the combustion surfaces but insufficient to decrease timing delay in the engine. The added dicyclopentadienyl iron may produce iron oxide on the engine cylinder surface (Fe2O3), which reacts with carbon deposits (soot) to form Fe and CO2, thereby removing the deposits. However, this method may accelerate the aging of the engine by formation of rust.
Valentine, et al., U.S. Patent Appl. Publ. No. 2003/0148235, the disclosure of which is incorporated herein by reference, describe specific bimetallic or trimetallic fuel-borne catalysts for increasing the fuel combustion efficiency. The catalysts reduce fouling of heat transfer surfaces by unburned carbon while limiting the amount of secondary additive ash which may itself cause overloading of particulate collector devices or emissions of toxic ultra fine particles when used in forms and quantities typically employed. By utilizing a fuel containing a fuel-soluble catalyst comprised of platinum and at least one additional metal comprising cerium and/or iron, production of pollutants of the type generated by incomplete combustion is reduced. Ultra low levels of nontoxic metal combustion catalysts can be employed for improved heat recovery and lower emissions of regulated pollutants. However, fuel additives of this type, in addition to using the rare and expensive metals such as platinum, can require several months before the engine is “conditioned”. By “conditioned” is meant that all the benefits of the additive are not obtained until the engine has been operated with the catalyst for a period of time. Initial conditioning may require 45 days and optimal benefits may not be obtained until 60-90 days. Additionally, free metal may be discharged from the exhaust system into the atmosphere, where it may subsequently react with living organisms.
Cerium dioxide is widely used as a catalyst in converters for the elimination of toxic exhaust emission gases and the reduction in particulate emissions in diesel powered vehicles. Within the catalytic converter, the cerium dioxide can act as a chemically active component, acting to release oxygen in the presence of reductive gases, as well as to remove oxygen by interaction with oxidizing species.
Cerium dioxide may store and release oxygen by the reversible process shown in equation 1.2CeO2←→Ce2O3+½O2  (eq. 1)The redox potential between the Ce3+ and Ce4+ ions lies between 1.3 and 1.8V and is highly dependent upon the anionic groups present and the chemical environment (CERIUM: A Guide to its Role in Chemical Technology, 1992 by Molycorp, Inc, Library of Congress Catalog Card Number 92-93444)). This allows the foregoing reaction to easily occur in exhaust gases. Cerium dioxide may provide oxygen for the oxidation of CO or hydrocarbons in an oxygen starved environment, or conversely may absorb oxygen for the reduction of nitrogen oxides (NOx) in an oxygen rich environment. Similar catalytic activity may also occur when cerium dioxide is added as an additive to fuel, for example, diesel or gasoline. However, for this effect to be useful, the cerium dioxide must be of a particle size small enough, i.e., nanoparticulate (<100 nm), to remain in a stable dispersion in the fuel. In addition, as catalytic effects depend on surface area, the small particle size renders the nanocrystalline material more effective as a catalyst. The incorporation of cerium dioxide in fuel serves not only to act as a catalyst to reduce toxic exhaust gases produced by fuel combustion, for example, by the “water gas shift reaction”CO+H2O->CO2+H2,but also to facilitate the burning off of particulates that accumulate in the particulate traps typically used with diesel engines.
Cerium dioxide nanoparticles are particles that have a mean diameter of less than 100 nm. For the purposes of this disclosure, unless otherwise stated, the diameter of a nanoparticle refers to its hydrodynamic diameter, which is the diameter determined by dynamic light scattering technique and includes molecular adsorbates and the accompanying solvation shell of the particle. Alternatively, the geometrical particle diameter may be estimated using transmission electron micrography.
Vehicle on-board dosing systems that dispense cerium dioxide into the fuel before it enters the engine are known, but such systems are complicated and require extensive electronic control to feed the appropriate amount of additive to the fuel. To avoid such complex on-board systems, cerium dioxide nanoparticles can also be added to fuel at an earlier stage to achieve improved fuel efficiency. They can, for example, be incorporated at the refinery, typically along with processing additives such as, for example, cetane improvers or added at a fuel distribution tank farm.
Cerium dioxide nanoparticles can also be added at a fuel distribution center, where it can be rack injected into large (˜100,000 gal) volumes of fuel or at a smaller fuel company depot, which would allow customization according to specified individual requirements. In addition, the cerium dioxide may be added at a filling station during delivery of fuel to a vehicle, which would have the potential advantage of improved stabilization of the particle dispersion.
Fuel additives, such as PuriNOx™ manufactured by Lubrizol Corporation, have been developed that are useful for the reduction of NOx and particulate material emissions, however, the composition of these fuel additives often includes 15-20% water. This “emulsified” fuel additive is commonly mixed with fuel at a level of 5-10%. The resulting high water content can lead to a loss in engine power and lower fuel economy. Thus it would be desirable to formulate a fuel additive that afforded reduction in nitrogen oxide and particulate material emissions, while simultaneously maintaining optimum engine performance.
Cerium nanoparticles and the associated free radical initiators incorporated into reverse-micellar compositions as described below, can provide a possible solution to this problem. Cerium nanoparticles may form a ceramic layer on the engine cylinders and moving parts essentially turning the engine into a catalytic device. Their catalytic efficiency derives from the fact that they provide a source of oxygen atoms during combustion by undergoing reduction according to the equation (1). This results in better fuel combustion and reduced levels of particulate material emissions. Additionally, when used as a fuel additive, these nanoparticles can provide improved engine performance by reducing engine friction. As an alternative mode of introduction, cerium dioxide nanoparticles can be added to the lube oil and act as a lubricity enhancing agent to reduce internal friction. This will also improve fuel efficiency.
Although substantially pure cerium dioxide nanoparticles are beneficially included in fuel additives, it may be of further benefit to use cerium dioxide doped with components that result in the formation of additional oxygen vacancies being formed. For this to occur, the dopant should be divalent or trivalent, i.e., a divalent or trivalent ion of an element that is a rare earth metal, a transition metal or a metal of Group IIA, IIIB, VB, or VIB of the Periodic Table, and of a size that allows incorporation of the ion in a lattice position within the surface or sub-surface region of the cerium dioxide nanoparticles. This substitutional ion doping is preferred to interstitial ion doping, where the dopants occupy spaces between the normal lattice positions.
The following publications, the disclosures all of which are incorporated herein by reference, describe fuel additives containing cerium oxidic compounds.
Hawkins et al., U.S. Pat. No. 5,449,387, discloses a cerium (IV) oxidic compound having the formula:(H2O)p[CeO(A)2(AH)n]m in which the radicals A, which are the same or different, are each an anion of an organic oxyacid AH having a pKa greater than 1, p is an integer ranging from 0 to 5, n is a number ranging from 0 to 2, and m is an integer ranging from 1 to 12. The organic oxyacid is preferably a carboxylic acid, more preferably, a C2-C20 monocarboxylic acid or a C4-C12 dicarboxylic acid. The cerium-containing compounds can be employed as catalysts for the combustion of hydrocarbon fuels.
Valentine et al., U.S. Pat. No. 7,063,729, discloses a low-emissions diesel fuel that includes a bimetallic, fuel-soluble platinum group metal and cerium catalyst, the cerium being provided as a fuel-soluble hydroxyl oleate propionate complex.
Chopin et al., U.S. Pat. No. 6,210,451, discloses a petroleum-based fuel that includes a stable organic sol that comprises particles of cerium dioxide in the form of agglomerates of crystallites (preferred size 3-4 nm), an amphiphilic acid system containing at least one acid whose total number of carbons is at least 10, and an organic diluent medium. The controlled particle size is no greater than 200 nm.
Birchem et al., U.S. Pat. No. 6,136,048, discloses an adjuvant for diesel engine fuels that includes a sol comprising particles of oxygenated compound having a d90 no greater than 20 nm, an amphiphilic acid system, and a diluent. The oxygenated metal compound particles are prepared from the reaction in solution of a rare earth salt such as a cerium salt with a basic medium, followed by recovery of the formed precipitate by atomization or freeze drying.
Lemaire et al., U.S. Pat. No. 6,093,223, discloses a process for producing aggregates of ceric oxide crystallites by burning a hydrocarbon fuel in the presence of at least one cerium compound. The soot contains at least 0.1 wt. % of ceric oxide crystallite aggregates, the largest particle size being 50-10,000 angstroms, the crystallite size being 50-250 angstroms, and the soot having an ignition temperature of less than 400° C.
Hazarika et al., U.S. Patent Appl. Publ. No. 2003/0154646, discloses a method of improving fuel efficiency and/or reducing fuel emissions of a fuel burning apparatus, the method comprising dispersing at least one particulate lanthanide oxide, particularly cerium dioxide, in the fuel, wherein the particulate lanthanum oxide is coated with a surfactant selected from the group consisting of alkyl carboxylic anhydrides and esters having at least one C10 to C30 alkyl group.
Collier et al., U.S. Patent Appl. Publ. No. 2003/0182848, discloses a diesel fuel composition that improves the performance of diesel fuel particulate traps and contains a combination of 1-25 ppm of metal in the form of a metal salt additive and 100-500 ppm of an oil-soluble nitrogen-containing ashless detergent additive. The metal may be an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB, VIIIB, IB, IIB, or any of the rare earth metals having atomic numbers 57-71, especially cerium, or mixtures of any of the foregoing metals.
Collier et al., U.S. Patent Appl. Publ. No. 2003/0221362, discloses a fuel additive composition for a diesel engine equipped with a particulate trap, the composition comprising a hydrocarbon solvent and an oil-soluble metal carboxylate or metal complex derived from a carboxylic acid containing not more than 125 carbon atoms. The metal may be an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB, VIIIB, IB, IIB, or a rare earth metal, including cerium, or mixtures of any of the foregoing metals.
Caprotti et al., U.S. Patent Appl. Publ. No. 2004/0035045, discloses a fuel additive composition for a diesel engine equipped with a particulate trap. The composition comprises an oil-soluble or oil-dispersible metal salt of an acidic organic compound and a stoichiometric excess of metal. When added to the fuel, the composition provides 1-25 ppm of metal, which is selected from the group consisting of Ca, Fe, Mg, Sr, Ti, Zr, Mn, Zn, and Ce.
Caprotti et al., U.S. Patent Appl. Publ. No. 2005/0060929, discloses a diesel fuel composition stabilized against phase separation that contains a colloidally dispersed or solubilized metal catalyst compound and 5-1000 ppm of a stabilizer that is an organic compound having a lipophilic hydrocarbyl chain attached to at least two polar groups, at least one of which is a carboxylic acid or carboxylate group. The metal catalyst compound comprises one or more organic or inorganic compounds or complexes of Ce, Fe, Ca, Mg, Sr, Na, Mn, Pt, or mixtures thereof.
Wakefield, U.S. Pat. No. 7,169,196 B2, discloses a fuel comprising cerium dioxide particles that have been doped with a divalent or trivalent metal or metalloid that is a rare earth metal, a transition metal, or a metal of Group IIa, IIIB, VB, or VIB of the Periodic Table.
Caprotti et al., U.S. Patent Appl. Publ. No. 2006/0000140, discloses a fuel additive composition that comprises at least one colloidal metal compound or species and a stabilizer component that is the condensation product of an aldehyde or ketone and a compound comprising one or more aromatic moieties containing a hydroxyl substituent and a further substituent chosen from among hydrocarbyl, —COOR, or —COR, R being hydrogen or hydrocarbyl. The colloidal metal compound preferably comprises at least one metal oxide, preferred oxides being iron oxide, cerium dioxide, or cerium-doped iron oxide.
Scattergood, International Publ. No. WO 2004/065529, discloses a method for improving the fuel efficiency of fuel for an internal combustion engine that comprises adding to the fuel cerium dioxide and/or doped cerium dioxide and, optionally, one or more fuel additives.
Anderson et al., International Publ. No. WO 2005/012465, discloses a method for improving the fuel efficiency of a fuel for an internal combustion engine that comprises lubricating oil and gasoline, the method comprising adding cerium dioxide and/or doped cerium dioxide to the lubricating oil or the gasoline.
Cerium-containing nanoparticles can be prepared by a variety of techniques known in the art. Regardless of whether the synthesized nanoparticles are made in a hydrophilic or hydrophobic medium, the particles normally require a stabilizer to prevent undesirable agglomeration. The following publications, the disclosures all of which are incorporated herein by reference, describe some of these synthetic techniques.
Talbot et al., U.S. Pat. No. 6,752,979, discloses a method of producing metal oxide particles having nano-sized grains that consists of: mixing a solution containing one or more metal cations with a surfactant under conditions such that surfactant micelles are formed within the solution, thereby forming a micellar liquid; and heating the micellar liquid to remove the surfactant and form metal oxide particles having a disordered pore structure. The metal cations are selected from the group consisting of cations from Groups 1A, 2A, 3A, 4A, 5A, and 6A of the Periodic Table, transition metals, lanthanides, actinides, and mixtures thereof. Preparations of particles of cerium dioxide and mixed oxides containing cerium and one or more other metals are included in the illustrative examples.
Chane-Ching et al., U.S. Pat. No. 6,271,269, discloses a process for preparing storage-stable organic sols that comprises: reacting a base reactant with an aqueous solution of the salt of an acidic metal cation to form an aqueous colloidal dispersion containing excess hydroxyl ions; contacting the aqueous colloidal dispersion with an organic phase comprising an organic liquid medium and an organic acid; and separating the resulting aqueous/organic phase mixture into an aqueous phase and a product organic phase. Preferred metal cations are cerium and iron cations. The colloidal particulates have hydrodynamic diameters in the range of 50-2000 angstroms.
Chane-Ching, U.S. Pat. No. 6,649,156, discloses an organic sol containing cerium dioxide particles that are made by a thermal hydrolysis process; an organic liquid phase; and at least one amphiphilic compound chosen from polyoxyethylenated alkyl ethers of carboxylic acids, polyoxyethylenated alkyl ether phosphates, dialkyl sulfosuccinates, and quaternary ammonium compounds. The water content of the sols may not be more than 1%. The mean crystallite size is about 5 nm, while the particle agglomerates of these crystallites range in size from 200 to 10 nm.
Chane-Ching, U.S. Pat. No. 7,008,965, discloses an aqueous colloidal dispersion of a compound of cerium and at least one other metal, the dispersion having a conductivity of at most 5 mS/cm and a pH between 5 and 8.
Chane-Ching, U.S. Patent Appl. Publ. No. 2004/0029978 (abandoned Dec. 7, 2005), discloses a surfactant formed from at least one nanoparticle that has amphiphilic characteristics and is based on a metal oxide, hydroxide and/or oxyhydroxide, on the surface of which organic chains with hydrophobic characteristics are bonded. The metal is preferably selected from among cerium, aluminum, titanium or silicon, and the alkyl chain comprises 6-30 carbon atoms, or polyoxyethylene monoalkyl ethers of which the alkyl chain comprises 8-30 carbon atoms and the polyoxyethylene part comprises 1-10 ethyoxyl groups. The particle is an isotopic or spherical particle having an average diameter of 2-40 nm.
Blanchard et al., U.S. Patent Appl. Publ. No. 2006/0005465, discloses an organic colloidal dispersion comprising: particles of at least one compound based on at least one rare earth, at least one acid, and at least one diluent, wherein at least 90% of the particles are monocrystalline. Example 1 describes the preparation of a cerium dioxide colloidal solution from cerium acetate and an organic phase that includes Isopar hydrocarbon mixture and isostearic acid. The resulting cerium dioxide particles had a d50 of 2.5 nm, and the size of 80% of the particles was in the range of 1-4 nm.
Zhou et al., U.S. Pat. No. 7,025,943, discloses a method for producing cerium dioxide crystals that comprises: mixing a first solution of a water-soluble cerium salt with a second solution of alkali metal or ammonium hydroxide; agitating the resulting reactant solution under turbulent flow conditions while concomitantly passing gaseous oxygen through the solution; and precipitating cerium dioxide particles having a dominant particle size within the range of 3-100 nm. In Example 1, the particle size is stated to be around 3-5 nm.
Noh et al., U.S. Patent Appl. Publ. No. 2004/0241070, discloses a method for preparing single crystalline cerium dioxide nanopowder comprising: preparing cerium hydroxide by precipitating a cerium salt in the presence of a solvent mixture of organic solvent and water, preferably in a ratio of about 0.1:1 to about 5:1 by weight; and hydrothermally reacting the cerium hydroxide. The nanopowder has a particle size of about 30-300 nm.
Chan, U.S. Patent Appl. Publ. No. 2005/0031517, discloses a method for preparing cerium dioxide nanoparticles that comprises: rapidly mixing an aqueous solution of cerium nitrate with aqueous hexamethylenetetramine, the temperature being maintained at a temperature no higher than about 320° K while nanoparticles form in the resulting mixture; and separating the formed nanoparticles. The mixing apparatus preferably comprises a mechanical stirrer and a centrifuge. In the illustrative example, the prepared cerium dioxide particles are reported to have a diameter of about 12 nm.
Ying et al., U.S. Pat. Nos. 6,413,489 and 6,869,584, disclose the synthesis by a reverse micelle technique of nanoparticles that are free of agglomeration and have a particle size of less than 100 nm and a surface area of at least 20 m2/g. The method comprises introducing a ceramic precursor that includes barium alkoxide and aluminum alkoxide in the presence of a reverse emulsion.
Illustrative example 9 of U.S. Pat. Nos. 6,413,489 and 6,869,584 describes the inclusion of cerium nitrate in the emulsion mixture to prepare cerium-doped barium hexaaluminate particles, which were collected by freeze drying and calcined under air to 500° C. and 800° C. The resulting particles had grain sizes of less than 5 nm and 7 nm at 500° C. and 800° C., respectively. Illustrative example 10 describes the synthesis of cerium-coated barium hexaaluminate particles. Following calcination, the cerium-coated particles had grain sizes of less than 4 nm, 6.5 nm, and 16 nm at 500° C., 800° C., and 1100° C., respectively.
A related publication, Ying et al., U.S. Patent Appl. Publ. No. 2005/0152832, discloses the synthesis, by a reverse micelle technique within an emulsion having a 1-40% water content, of nanoparticles that are free of agglomeration and have a particle size of less than 100 nm. The nanoparticles are preferably metal oxide particles, which can be used to oxidize hydrocarbons.
Illustrative Examples 9 and 10 of U.S. Patent Appl. Publ. No. 2005/0152832 describe the preparation of, respectively, cerium-doped and cerium-coated barium hexaaluminate particles. Example 13 describes the oxidation of methane with the cerium-coated particles.
Hanawa et al., U.S. Pat. No. 5,938,837, discloses a method for preparing cerium dioxide particles, intended primarily for use as a polishing agent, that comprises mixing, with stirring, an aqueous solution of cerous nitrate with a base, preferably aqueous ammonia, in such a mixing ratio that the pH value of the mixture ranges from 5 to 10, preferably 7 to 9, then rapidly heating the resulting mixture to a temperature of 70-100° C., and maturing the mixture of cerous nitrate with a base at that temperature to form the grains. The product grains are uniform in size and shape and have an average particle size of 10-80 nm, preferably 20-60 nm.
European Patent Application EP 0208580, published 14 Jan. 1987, inventor Chane-Ching, applicant Rhone Poulenc, discloses a cerium (IV) compound corresponding to the general formulaCe(M)x(OH)y(NO3)2 wherein M represents an alkali metal or quaternary ammonium radical, x is between 0.01 and 0.2, y is such that y=4−z+x, and z is between 0.4 and 0.7. A process for preparing a colloidal dispersion of the cerium (IV) compound produces particles with a hydrodynamic diameter between about 1 nm and about 60 nm, suitably between about 1 nm and about 10 nm, and desirably between about 3 nm and 8 nm.
The doping of cerium dioxide with metal ions (reported as early as 1975) and the resultant dopant effects on the electronic and oxygen diffusion properties are well described by Trovarelli, Catalysis by Ceria and Related Materials, Catalytic Science Series, World Scientific Publishing Co., 37-46 (2002) and references cited therein.
S. Sathyamurthy et al., Nano Technology 16, (2005), pp 1960-1964, describes the reverse micellar synthesis of CeO2 from cerium nitrate, using sodium hydroxide as the precipitating agent and n-octane containing the surfactant cetyltrimethylammonium bromide (CTAB) and the cosurfactant 1-butanol as the oil phase. The resulting polyhedral particles had an average size of 3.7 nm, but the reaction would be expected to proceed in low yield.
S. Seal et al., Journal of Nano Particle Research, (2002), p 438, describes the preparation from cerium nitrate and ammonium hydroxide of nanocrystalline ceria particles for a high-temperature oxidation-resistant coating using an aqueous microemulsion system containing AOT as the surfactant and toluene as the oil phase. The ceria nanoparticles formed in the upper oil phase of the reaction mixture had a particle size of 5 nm.
Pang et al., J. Mater. Chem., 12 (2002), pp 3699-3704, prepared Al2O3 nanoparticles by a water-in-oil microemulsion method, using an oil phase containing cyclohexane and the non-ionic surfactant Triton X-114, and an aqueous phase containing 1.0 M AlClO3. The resulting Al2O3 particles, which had a particle size of 5-15 nm, appeared to be distinctly different from the hollow ball-shaped particles of submicron size made by a direct precipitation process.
Cuif et al, U.S. Pat. No. 6,133,194, the disclosure of which is incorporated herein by reference, describes a process that comprises reacting a metal salt solution containing cerium, zirconium, or a mixture thereof, a base, optionally an oxidizing agent, and an additive selected from the group consisting of anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids, and carboxylate salts, thereby forming a product. The product is subsequently calcined at temperatures >500 C (which would effectively carbonize the claimed surfactants).
Hazbun et al., U.S. Pat. No. 4,744,796, the disclosure of which is incorporated herein by reference, describes a microemulsion fuel composition that includes a hydrocarbon fuel and a cosurfactant combination of t-butyl alcohol and at least one amphoteric, anionic, cationic, or nonionic surfactant. Preferred surfactants are fatty acids or fatty acid mixtures.
Hicks et al., U.S. Patent Appl. Publ. No. 2002/0095859, the disclosure of which is incorporated herein by reference, describes additive compositions for liquid hydrogen fuels that include one or more surfactants selected from the group consisting of amphoteric, anionic, cationic, or nonionic surfactants, and optionally one or more cosurfactants selected from the group consisting of alcohols, glycols, and ethers.
As described previously, various methods and apparatus have been reported for preparing cerium nanoparticles including those described by Chane-Ching, et al., U.S. Pat. No. 5,017,352; Hanawa, et al., U.S. Pat. No. 5,938,837; Melard, et al., U.S. Pat. No. 4,786,325; Chopin, et al., U.S. Pat. No. 5,712,218; Chan, U.S. Patent Appl. Publ. No. 2005/0031517; and Zhou, et al., U.S. Pat. No. 7,025,943, the disclosures of which are incorporated herein by reference. However, current methods do not allow the economical and facile preparation of cerium nanoparticles in a short period of time at very high suspension densities (greater than 0.5 molal, i.e., 9 wt %) that are sufficiently small in size (less than 8 nm in mean diameter), uniform in size frequency distribution (coefficient of variation [COV] of less than 15%, where COV is the standard deviation divided by the mean diameter), and stable for many desirable applications.
A typical chemical reactor that might be used to prepare cerium dioxide includes a reaction chamber that includes a mixer (see, for example, FIG. 1 in Zhou et al. U.S. Pat. No. 7,025,943). A mixer typically includes a shaft, and propeller or turbine blades attached to the shaft, and a motor that turns the shaft, such that the propeller is rotated at high speed (1000 to 5000 rpm). The shaft can drive a flat blade turbine for good meso mixing (micro scale) and a pitched blade turbine for macro mixing (pumping fluid through out the reactor).
Such a device is described in Antoniades, U.S. Pat. No. 6,422,736, entitled “Scaleable Device Impeller Apparatus For Preparing Silver Halide Grains.” This type of reactor is useful for fast reactions such as that shown by the equation below, wherein the product, AgCl, is a crystalline material having a diameter on the order of several hundred nanometers up to several thousand nanometers.AgNO3+NaCl→AgCl+NaNO3 
Cerium dioxide particles prepared using this type of mixing are often too large to be useful for certain applications. It is highly desirable to have the smallest cerium dioxide particles possible as their catalytic propensity (ability to donate oxygen to a combustion system, i.e., equation 1) increases with decreasing particle size, especially for particles having a mean diameter of less than 10 nm.
A schematic example of a batch reactor that can be used to produce cerium dioxide nanoparticles is shown in FIG. 1. The reactor (10) includes inlet ports (11, and 12) for adding reactants, a propeller, shaft, and motor, 15, 14, and 13, for mixing. The reaction mixture 18 is contained in a reactor vessel 16. Addition of reactants, such as cerium nitrate, an oxidant, and hydroxide ion, can result in the formation of nanoparticles. The particles initially form as very small nuclei. Mixing causes the nuclei to circulate, shown by the dashed arrows (17) in FIG. 1. As the nuclei continuously circulate through the reactive mixing regime they grow (increase in diameter) as they incorporate fresh reactants. Thus, after an initial steady state concentration of nuclei is formed, this nuclei population is subsequently grown into larger particles in a continuous manner. This nucleation and growth process is not desirable if one wishes to limit the final size of the particles while still maintaining a high particle suspension density. Such a batch reactor is not useful for producing a high yield (greater than 1 molal) of cerium dioxide nanoparticles that are very small, for example, less than 10 nm in a reasonably short reaction time (for example, less than 60 minutes).
An example of this nucleation and growth process applied to the aqueous precipitation of CeO2 is the work of Zhang et al., J. Appl. Phys., 95, 4319 (2004) and Zhang, et al., Applied Physics Letters, 80, 127 (2002). Using cerium nitrate hexahydrate as the cerium source (very dilute at 0.0375M) and 0.5 M hexamethylenetetramine as the ammonia precursor, 2.5 to 4.25 nm cerium dioxide particles are formed in times that are less than 50 minutes. These particles are subsequently grown to 7.5 nm or greater using reaction times on the order of 250 minutes (or 600 minutes depending upon growth conditions). The limitations of particle size, concentration and reaction time would exclude this process from consideration as an economically viable route to bulk commercial quantities of CeO2 nanoparticles.
I. H. Leubner, Current Opinion in Colloid and Interface Science, 5, 151-159 (2000), Journal of Dispersion Science and Technology, 22, 125-138 (2001) and ibid. 23, 577-590 (2002), and references cited therein, provides a theoretical treatment that relates the number of stable crystals formed with molar addition rate of reactants, solubility of the crystals and temperature. The model also accounts for the effects of diffusion, kinetically controlled growth processes, Ostwald ripening agents and growth restrainers/stabilizers on crystal number. High molar addition rates, low temperatures, low solubility, and the presence of growth restrainers all favor large numbers of nuclei and consequently smaller final grain or particle size
In contrast to batch reactors, colloid mills typically have flat blade turbines turning at 10,000 rpm, whereby the materials are forced through a screen whose holes can vary in size from fractions of a millimeter to several millimeters. Generally, no chemical reaction is occurring, but only a change in particle size. In certain cases, particle size and stability can be controlled thermodynamically by the presence of a surfactant. For example, Langer et al., in U.S. Pat. No. 6,368,366 and U.S. Pat. No. 6,363,237, incorporated herein by reference, describe an aqueous microemulsion in a hydrocarbon fuel composition made under high shear conditions. However, the aqueous particle phase (the discontinuous phase in the fuel composition) has a large size, on the order of 1000 nm.
Colloid mills are not useful for reducing the particle size of large cerium dioxide particles because the particles are too hard to be sheared by the mill in a reasonable amount of time. The preferred method for reducing large, agglomerated cerium dioxide particles from the micron size down into the nanometer size is milling for several days on a ball mill in the presence of a stabilizing agent. This is a time consuming, expensive process that invariably produces a wide distribution of particle sizes. Thus, there remains a need for an economical and facile method to synthesize large quantities (at high suspension densities) of very small nanometric particles of cerium dioxide with a uniform size distribution.
Aqueous precipitation may offer a convenient route to cerium nanoparticles. However, to be useful as a fuel-borne catalyst for fuels, cerium dioxide nanoparticles must exhibit stability in a nonpolar medium (for example, diesel fuel). Most stabilizers used to prevent agglomeration in an aqueous environment are ill suited to the task of stabilization in a nonpolar environment. When placed in a nonpolar solvent, such particles tend to immediately agglomerate and, consequently, lose some, if not all, of their desirable nanoparticulate properties. Thus it would be desirable to form stable cerium dioxide particles in an aqueous environment, retain the same stabilizer on the particle surface, and then be able to transfer these particles to a nonpolar solvent, wherein the particles would remain stable and form a homogeneous mixture. In this simplified and economical manner, one could eliminate the necessity for changing surface stabilizer's affinity from polar to non-polar. Changing stabilizers can involve a difficult displacement reaction or separate, tedious isolation-redispersal methods (for example, precipitation and subsequent redispersal with the new stabilizer using ball milling).
Thus, there remains a need for an efficient and economical method to synthesize stable cerium dioxide nanoparticles in a polar, aqueous environment, and then transfer these particles to a non-polar environment wherein a stable homogeneous mixture is formed.
For some applications, it may even be desirable to have some relatively low level of water present during the combustion process of an internal combustion engine. The previously mentioned, Hicks et al., U.S. Patent Appl. Publ. No. 2002/0095859 suggests that as little as 5 to 95 ppm water (as a microemulsion) improves hydrocarbon fuel combustion via the reduction of cyclic dispersion (variability between compression cycles).
Water added to diesel fuel is thought to improve combustion in three ways:                1. Water promotes a finer, more even spray pattern for more complete combustion.        2. Water lowers the combustion temperature to reduce nitrous oxide emissions (flame temperature of 2900° F.).        3. Water delays combustion slightly to reduce particulate emissions.        
J. Ying et al in WO 98/18884 describe a thermally and temporally stable water-in-fuel emulsion having micelle size of <100 nm and including water in an amount of at least 8 wt. percent. As there was no attendant measurement of engine power, the claimed 85-90% reductions in particulate emissions may have been an artifact of the loss of engine power and thus been an unacceptable trade-off of power for emissions reduction. Fuel additives that include cerium dioxide nanoparticles, wherein nanoparticles typically have a mean diameter of 100 nm or less, stabilized with a surfactant, such as sodium dodecyl succinate, and optionally containing copper, are known. These types of fuel additives also have a long conditioning period.
The use of cerium nanoparticles to provide a high temperature oxidation resistant coating has been reported, for example, see “Synthesis Of Nano Crystalline Ceria Particles For High Temperature Oxidization Resistant Coating,” S. Seal et al., Journal of Nanoparticle Research, 4, 433-438 (2002). The deposition of cerium dioxide on various surfaces has been investigated, including Ni, chromia and alumina alloys, and stainless steel and on Ni, and Ni—Cr coated alloy surfaces. It was found that a cerium dioxide particle size of 10 nm or smaller is desirable. Ceria particle incorporation subsequently inhibits oxidation of the metal surface.
In addition, the extent to which CeO2 can act as a catalytic oxygen storage material, described by equation 1, is governed in part by the CeO2 particle size. At 20 nm particle sizes and below, the lattice parameter increases dramatically with decreasing crystallite size (up to 0.45% at 6 nm, see for example Zhang, et al., Applied Physics Letters, 80 1, 127 (2002)). The associated size-induced lattice strain is accompanied by an increase in surface oxygen vacancies that results in enhanced catalytic activity. This (inverse) size dependent activity provides not only for more efficient fuel cells, but better oxidative properties when used in the combustion of petroleum fuels.
Henly, U.S. Patent Appl. Publ. No. 2005/0005506, the disclosure of which is incorporated herein by reference, has described a distillate fuel additive composition, including calcium sulfonate detergent, a succinimide dispersant, and an organomanganese compound. The organic manganese compound, along with other compounds, acts to improve the cleanliness of the fuel system.
Rim, U.S. Pat. No. 6,892,531, the disclosure of which is incorporated herein by reference, describes an engine lubricating oil composition for a diesel engine that includes a lubricating oil and 0.05-10 wt. % of a catalyst additive comprising cerium carboxylate.
As described above, currently available fuel additives have improved the performance of diesel engines; however further improvements are still needed. It would be desirable to formulate a fuel additive for diesel engines that provides: improved fuel combustion while maintaining engine power while simultaneously reducing PM emissions. In addition, protection of engines from wear, reduced engine friction, greater lubricity, with improved fuel efficiency would be tremendously beneficial. It would also be desirable to provide one or more of these features without requiring a long conditioning period.